Abstract 105: MCL-1 Promotes Drp1-Mediated Mitochondrial Fission as an Adaptive Response to Stress

Abstract

The anti-apoptotic BCL-2 family member, Myeloid Cell Leukemia-1 (MCl-1), is highly expressed in myocardium and plays a critical role in maintaining mitochondrial homeostasis. We have previously found that cardiomyocyte-specific MCL-1 knockout mice develop rapid cardiac dysfunction and cardiomyopathy but show little activation of apoptotic cell death. Instead, loss of MCL-1 resulted in atypical mitochondrial morphology and function. This suggests that besides its anti-apoptotic role, MCL-1 may have broader functions in regulating mitochondrial dynamics and function. MCL-1 localizes to two different mitochondrial locations in myocytes. One form exists on the outer mitochondrial membrane (MCL-1 OM ) and a shorter cleaved form resides in the mitochondrial matrix (MCL-1 Matrix ). We found that overexpression of MCL-1 WT or MCL-1 OM , but not MCL-1 Matrix , induces fragmentation and perinuclear aggregation of the mitochondria in a Drp1-dependent manner. Mutating MCL-1’s BH3 domain (G198E D199A), which is required for MCL-1’s anti-apoptotic function, completely abrogates its ability to induce perinuclear aggregation. Interestingly, a MCL-1-BCL-2 chimera, in which MCL-1’s BH domains are replaced with those of BCL-2, is still able to induce perinuclear aggregation. This suggests that the presence of a functional BH3 domain is sufficient for induction of perinuclear aggregation. We confirmed that there is increased interaction between endogenous MCL-1 and Drp1 in response to a variety of fission-promoting stimuli, including glucose deprivation, hypoxia, and treatment with rotenone or FCCP. MCL-1 overexpression also protects against cell death in response to these stimuli, but this protection is abrogated when Drp1 is knocked down using siRNA. Additionally, Drp1 levels are significantly increased at the mitochondria in the hearts of MCL-1 OM transgenic mice, and this increase corresponds to an increased interaction between MCL-1 and Drp1 in these transgenic mice. Consistent with these findings, many of the mitochondria in these transgenic mice appear to be smaller than those from the WT mice, indicative of enhanced mitochondrial fission. Thus, our data suggests that MCL-1 functions as a positive regulator of Drp1-mediated mitochondrial fission.